Field of the Invention
The present invention relates to a wavefront measurement method which can be used to evaluate an optical element. The present invention also relates to a shape measurement method, an optical element manufacturing method, an optical apparatus manufacturing method, a program, and a wavefront measurement apparatus.
Description of the Related Art
In recent years, optical apparatuses, such as cameras, optical drives, and exposure apparatuses, including aspherical optical elements (mirrors, lenses, etc.) to reduce the size of optical systems installed therein have come into widespread use. To achieve efficient production of high-quality aspherical optical elements, a measurement technology for readily evaluating the shapes of the aspherical optical elements is required.
One well-known example of such a measurement technology is a measurement method using a Shack-Hartmann sensor including a microlens array and an image pickup element. When light is incident on an object, such as an aspherical optical element, and is reflected by the object, the reflected light travels as light (object light) having a wavefront that matches the shape of the object. The wavefront can be readily measured by detecting the object light with a Shack-Hartmann sensor, and the shape of the object can be determined from the measurement data of the wavefront.
When the object light is incident on the microlens array of the Shack-Hartmann sensor, a spot image including a plurality of spots are formed on the image pickup element. The spot image is captured, and the positions of the spots are detected. Incident angles of light rays incident on respective microlenses are calculated from the detected spot positions, and data of the wavefront of the object light can be readily calculated from the incident angle distribution of the light rays.
To increase the measurement resolution of the Shack-Hartmann sensor, it is necessary to reduce the pitch of the microlens array. However, when light is incident on each of the microlenses included in the microlens array, diffracted light is generated at the edges of the microlenses. Therefore, when the pitch is reduced, the diffracted light generated by the adjacent microlenses interferes with the spots, and the spot image will be distorted. Consequently, the wavefront measurement accuracy will be reduced. Therefore, to increase both the resolution and accuracy of the Shack-Hartmann sensor, it is necessary to reduce the error caused by the diffracted light generated by the adjacent microlenses.
Japanese Patent Laid-Open No. 2002-198279 describes a technology for reducing the influence (crosstalk) between the adjacent spots. According to Japanese Patent Laid-Open No. 2002-198279, first, an image of light incident on an object is captured by a wavefront aberration measurement apparatus, which includes a microlens array and an image pickup element. Thus, a light intensity distribution J0 of each spot image is obtained. The position P0 of each of the plurality of spots is calculated so that it can be expressed by a general formula P0=G(J0) by using a known measurement method G, such as a centroid method. Also, image data J1 based on the light intensity distribution expected to be obtained when it is assumed that the correct spot image is formed at the calculated positions P0 are obtained. More specifically, on the basis of an algorithm F based on a predetermined optical model, such as the microlens array, J1=F(P0) is approximated to a SINC function (=(Sin x)/x). Then, comparative spot image positions CP are calculated as CP=G(J1) on the basis of the determined spot image J1 by using the above-described calculation method G again. After that, the spot positions P0 are corrected in accordance with the differences between the acquired positions P0 and the calculated positions CP. As a result, the influence (crosstalk) between the adjacent spots can be reduced.
However, with the technology described in Japanese Patent Laid-Open No. 2002-198279, the image data J1 is expressed by using a SINC function, with which the travelling direction of the diffracted light generated when the light passes through the microlenses cannot be accurately expressed. Therefore, the diffracted light that travels in a direction different from the direction of the above-described light rays is not appropriately expressed. As a result, the detection error due to the diffracted light generated by the adjacent microlenses cannot be sufficiently reduced.
Japanese Patent No. 4212472 proposes a configuration in which rectangular masks are evenly arranged along a microlens array such that axes thereof are rotated by 25° with respect to the direction of the adjacent spots. In this case, priority axes (X1, Y1, X2, and Y2) of the diffracted light extend in directions different from the directions of the adjacent spots and the next spots, so that the diffracted light does not overlap these spots.
However, when a wavefront with a large degree of asphericity is incident on the Shack-Hartmann sensor, the spots are displaced by large amounts and move to positions close to the adjacent spots. Therefore, there is a risk that the detection error cannot be sufficiently reduced. Thus, with the technology described in Japanese Patent No. 4212472, a wavefront with a large degree of asphericity cannot always be accurately measured.
The present invention provides a wavefront measurement method using a high-resolution, high-accuracy Shack-Hartmann sensor with which spot detection error due to diffracted light generated by adjacent microlenses is suppressed. The present invention also provides a shape measurement method, an optical element manufacturing method, an optical apparatus manufacturing method, a program, and a wavefront measurement apparatus.